Mr Sayers claimed the defendant stood to lose his £80,000 job as a bodyguard in Iraq, but the killing last July offered a chance to resolve financial problems and an opportunity to bring up his other, healthy son.

He said: "Andrew Wragg cracked on that final afternoon."

Mr Sayers said the father was "transfixed with the compulsion" to end his son's final degradation.

He added: "Whatever happened, happened out of compassion."

Andrew Wragg told police his son's death had been a mercy killing

He said it was "crystal clear" the defendant had an abnormality of mind.

In his closing argument, prosecutor Philip Katz QC said: "Mercy killing is a highly emotive label and normally applied to a situation where the person who is killed is really at death's door and may have been for quite a long time.

"No one would suggest that, however poorly Jacob was, he was in that situation.

"He wasn't hospitalised, he wasn't receiving oxygen, he wasn't really in that palliative state... he was making his own decisions."

He said the test for jurors would be whether Mr Wragg had an abnormality of mind that would substantially impair responsibility for his actions.